Schistosome infections are studied quantitatively in man and experimental animals. Present studies emphasize fibrosis induced by schistosomiasis and resistance to reinfection with S. mansoni in laboratory mice. Initial studies show the fibrotic response to eggs in the liver of S. japonicum infected rabbits is much more marked than in the intestine. Mouse strains show marked differences in hepatic fibrosis. This has not yet been shown to have a genetic basis.